Hi,
It should work
lv_obj_t * obj = lv_btnmatrix_create(lv_scr_act());
lv_btnmatrix_set_btn_ctrl_all(obj, LV_BTNMATRIX_CTRL_CHECKABLE);
lv_btnmatrix_set_one_checked(obj, true);
/*Make a button checked manually*/
lv_btnmatrix_set_btn_ctrl(obj, 3, LV_BTNMATRIX_CTRL_CHECKED);
/*Uncheck all buttons*/
lv_btnmatrix_clear_btn_ctrl_all(obj, LV_BTNMATRIX_CTRL_CHECKED);